The aviation authorities of many countries require that commercial aircraft, and also military aircraft, when in civilian airspace on non-covert activities, have navigational lighting to improve their visibility at night. Under U.S. regulations, when any aircraft (military or civilian) is flown during darkness in unrestricted airspace or in a military operations area (MOA), the external lighting of the aircraft must conform to FAA requirements for chromaticity (‘color’), luminous intensity (‘brightness’), and angular coverage. Generally, the navigation lighting comprises red lights on the left side and green lights on the right side of the plane, and this external lighting normally is provided by incandescent light bulbs in sockets on the outside of the plane, conventionally powered by electricity from the internal electronics of the aircraft.
The lifetime of incandescent lamps, especially in military aircraft exterior lighting applications, is limited, and replacement of incandescent lamps in the exterior lighting fixtures of vehicles, in particular aircraft, is a frequent maintenance task. For example, interviews of squadron maintenance personnel indicate that the average lifetime of incandescent lamps in the main left and right navigation light fixtures in F-16 aircraft is approximately 75 hours. A means of increasing the reliability of exterior light sources while maintaining levels of luminous intensity required to meet Federal Aviation Administration (FAA) regulations is therefore needed.
Compared to conventional incandescent lamps, light emitting diodes (LEDs) contain no filaments and can theoretically provide lifetimes measured in thousands of hours. Also, LEDs are far more efficient in converting electrical energy into light energy. However, LEDs are not perfect converters of electrical energy into light energy, and some energy always will be lost as waste heat.
This creates a problem for possible use of LEDs in aircraft navigation lighting, because the heat created increases the temperature of the LEDs, and, for a given drive voltage, the hotter an LED gets, the less light that LED emits. The problem is even greater in the area of navigation light aviation applications, because to achieve luminous intensity levels conforming to FAA/ICAO regulations a number of high intensity LED diodes (individual light emitters) must be integrated and co-packaged. However, the added LEDs generate substantial heat, which elevates the temperature of the LEDs. This elevated temperature in turn tends to reduce the intensity of the light. Sufficiently high temperatures will ultimately degrade the LEDs so that they either go out entirely, or function at a greatly reduced illumination level. Use of LEDs in aircraft light systems is consequently subject to problems of overheating.
As mentioned above, military aircraft are required to have visible navigation lights similar to those of civilian and commercial aircraft. When flying a wartime night mission or night training sortie, a military aircraft may transit through unrestricted airspace in which civilian aircraft also operate, and it must have aircraft external lighting that conforms to FAA requirements during this transit. However, during night flight operations in wartime conditions, or during night flight training in restricted airspace, aircraft external lighting that is visible to the unaided eye is undesirable.
For military aircraft in covert activities or other military situations where visibility would be a disadvantage, one approach was for the pilot of the military aircraft to simply turn off the external lighting. The pilot can adjust the intensity of the navigation lights, or turn them off completely, with a brightness control dial in the cockpit hat varies the voltage of the AC current sent to the light socket.
In recent years, however, it has been noted that, in covert activities, while the aircraft was not visible to the enemy, it was also not visible to friendly aircraft, and planes began to be supplied with covert mode IR light sources in addition to the visible navigational lights. In covert operation, only emissions in the near-infrared of appropriate intensity are used, and visible navigation light is not emitted. The IR light emitted is not visible to the unaided human eye, but can be seen with appropriate viewing equipment, e.g., night vision goggles (NVGs) that are utilized during night operations in many military aircraft, which are very sensitive to the deep red and near-infrared region of the spectrum.
To upgrade to covert IR capability making military aircraft external lighting to be selectable between visible and covert modes at will during flight, IR light sources have ordinarily been additional arrays of IR diodes added to the outside of the plane in addition to the existing navigational lights. Alternatively, filters have been mounted over the existing navigational lights and IR diodes mounted in the light bulb fixtures. These kinds of additions, however, require substantial structural work to create the mounts and to wire the new fixtures into the aircraft body, which usually does not have very much extra room for more wiring. In addition, the extensive modifications result in considerable expense for an upgrade to covert IR capability.